The invention relates to a photo-thermal sensor for determining the concentration of a compound in a material sample.
To determine the concentration of a compound in a material sample, the sample is irradiated by light. If the ample contains a compound which absorbs the light, light energy is converted into heat energy, whereby the temperature of the sample rises. Since the refraction index of a material generally rises with the temperature, an optical medium corresponding to a gradient index lens is formed thereby, which is defined as a "thermal lens" T.L.). The presence of a thermal lens can be detected by directing a second light beam onto the sample and determining its refraction. The sample needs to be sufficiently transparent so that the second light beam can be detected after passage through the sample.
A photo-thermal sensor of this type is disclosed in a publication by Dorys Rojas, Robert J. Silva, Jonathan D. Spear and Richard E. Russo in Anal. Chem. 1991, 63, 1927-1932. With the sensor described therein the concentration of samples consisting of an aqueous solution of Nd.sup.+3 -ions in a cuvette is determined.
As excitation light source a color laser is utilized which emits a coherent light of a wave length of 590 nm and which is pumped by an argon ion laser. The light beam of the excitation light source is modulated by 10 Hz and is guided by means of a light conductor and by means of a chromatic optical arrangement comprising three lenses in such a way that at the sample location the beam is constricted. As probe laser beam a helium-neon laser beam is utilized whose coherent light beam extends in a direction normal to that of the excitation laser and is guided in an additional lens in such a way that a second constriction is formed in the beam after passage through the sample. The light of the excitation light source is deflected on a beam divider in the direction toward the sample. A part of 50% of the light which passes through the beam divider is directed to a first detector. The excitation laser beam passes through the beam divider and also strikes the sample. In the path of the light beam which passes through the sample there is arranged first an interference filter and then a second light conductor at which most of the light is diaphragmed out. Following this arrangement there is a second detector and an analyzing unit.
As is apparent from this publication the distance between the sample and the entry surface of the second light conductor must be at least 10-15 cm in order to provide for sufficient light intensity modulation to the second detector. Consequently such a photo-thermal sensor cannot be of very compact design.
Another sensor for the photo-thermal spectroscopy is known from DE 39 37 905 Cl. In this sensor, a sample light beam and a modulated excitation light beam are coupled into a light conductive fiber. At the end of the optical fiber, sample light beam and excitation light beam emerge and are focused in a chromatic lens arrangement. For this purpose a chromatic lens arrangement is utilized in which the two light beams have spatially distinct focal points. The medium to be analyzed is disposed in the area of the focal points. The intensity of the probe light beam is recorded by the detector by way of an apertured diaphragm. The detector signal is supplied to a central analyzing unit.
The use of a wave-length dependent lens arrangement in the particular sensor results in a limited choice of wave length combinations for the excitation light source and the probe laser. Because of the particular characteristics of representation of the optical fiber such a sensor is not particularly sensitive.
A further photo-thermal sensor is known from a publication of Shaole Wu and Norman J. Dovichi entitled "Fresnel diffraction theory for steady-state thermal lens measurements in thin films", J. Appl. Phys. 67(3) (1. Feb. 1990) pages 1170-1182. In this sensor, excitation light source and probe laser light source are the same. For signal optimization the Fresnel infraction laws are applied.
It is the object of the present invention to provide a photo-thermal sensor which is very compact. Particularly, the distance between the sample and the arrangement where most of the light passing through the sample is diaphragmed out is reduced without a loss of sensitivity with respect to the change of the refraction index.